There are two major arms to the immune system, supported by different types of cells called B-lymphocytes and T-lymphocytes (B-cells and T-cells). B cells make antibodies when they encounter antigens and, in most instances, these antibodies are protective. In autoimmune diseases, however, some of the antibodies react with the individual's tissues. When they deposit in tissue, they cause an inflammatory reaction and tissue damage. T-cells, like B-cells, are also activated when they encounter an antigen. As T-cells develop they undergo a process called “thymic education.” During thymic education, more than 95% of the T-cells die. The T-cells that have had a T-cell receptor that can recognize and react with the individuals's own tissues (self-antigens) are specifically eliminated. Some autoreactive T-cells escape the elimination process, however, and can initiate an immune response that results in autoimmune disease.
The modulation of T-cell activity remains a significant therapeutic goal in diseases with immunopathological T-cells. The fate of T lymphocytes following T cell receptor (TCR) stimulation is guided by the integration of costimulatory and inhibitory receptor inputs. Costimulatory ligands on antigen-presenting cells (APC) trigger cognate receptor molecules on T cells, with resultant enhancement of T cell proliferation, cytokine secretion, and differentiation. In contrast, binding of inhibitory ligand molecules to cognate counter-receptors on T lymphocytes diminishes effector functioning by inducing T cell unresponsiveness or programmed cell death (PCD) (also referred to as apoptosis). Costimulatory and inhibitory receptor pathway interactions are suggested by experiments demonstrating increased inhibitor activity in the presence of costimulator blockade.
Cytotoxic T lymphocyte-associated protein-4 (CTLA-4 (CD152)) is an inhibitory receptor molecule that is expressed on the surface of activated T lymphocytes. Following engagement with the B7-1 (CD80) and/or B7-2 (CD86) ligands resident on APC, the CTLA-4 counter-receptor, via associated SHP-2 phosphatase, inhibits T cell activation. On activated T cells, CTLA-4 exists as disulfide-linked homodimeric glycoprotein complexes. A recombinant, soluble CTLA-4:immunoglobulin G (CTLA-4:Ig) chimeric protein demonstrates inhibitory function by competitively blocking CD80/CD86 molecule binding to the activating CD28 acceptor on T cell surfaces. CTLA-4:Ig also exhibits immunosuppressive activity in animal models of graft rejection and autoimmune disease by blocking T cell costimulation through CD28. In addition, intracellular T cell survival signaling through CD28 is antagonized by APC treatment with CTLA-4:Ig, which can increase susceptibility to Fas-dependent PCD. The action of CTLA-4, as well as CTLA-4:Ig fusion proteins, are discussed in U.S. Pat. Nos. 5,885,776; 5,885,579; 5,851,795; and 5,968,510.
Apoptosis (or PCD) is a distinct form of cell death which is essential for the regulation of cellular homeostasis. In the immune system, Fas (CD95) receptor and its ligand, FasL (CD95L), participate in various processes involved in the induction of apoptosis, including immune cell-mediated cytotoxicity, and in the regulation of cellular immune responses. FasL is a member of the tumor necrosis factor superfamily and is expressed by a restricted subset of immune cells, including monocytes, NK cells, and activated B and T cells. On the cell surface, FasL is oriented as a type II membrane protein within trimeric complexes. Metalloproteinase cleavage of membrane-associated FasL releases soluble FasL (sFasL) trimers from the membrane. The FasL molecule triggers Fas-dependent PCD.
The valency of a molecule or molecular complex can be increased by association with the cell surface. Different coding sequences of recombinant sFasL molecules affect macromolecular aggregation and, in turn, affect sFasL pro-apoptotic function. In particular, a naturally processed sFasL molecule forms trimers and poorly induces apoptosis. In contrast, a recombinant full-length extracellular domain sFasL polypeptide forms higher order aggregates and displays highly potent apoptotic activity. Furthermore, complexes of sFasL produced by recombinant expression in human 293 cells require cross-linking for lysis of Fas-sensitive cells.
U.S. Pat. No. 5,830,469 discloses monoclonal antibodies and binding proteins that specifically bind to human Fas antigen; some of the antigens and antibodies are reported as stimulating T cell proliferation, inhibiting of anti-Fas CH-11 monoclonal antibody-mediated lysis of cells, and blocking Fas ligand-mediated lysis of cells. Fas-Fc fusion proteins are also disclosed.
U.S. Pat. Nos. 5,242,687; 5,601,828; and 5,623,056 disclose various fusion proteins containing a CD8 component that bind to a cell but do not mask a signal produced by the cell.
U.S. Pat. No. 5,359,046 discloses chimeric proteins comprised of an extracellular domain capable of binding to a ligand in a non-MHC restricted manner, a transmembrane domain and a cytoplasmic domain capable of activating a signaling pathway. Similar technology is disclosed in U.S. Pat. No. 5,686,281.